Along with high integration in LSIs, circuit line widths in semiconductor devices are becoming increasingly fine year by year. To form a desired circuit pattern on a semiconductor device, a method is used in which a high-precision original drawing pattern (also referred to as a mask or as a reticle particularly when the original drawing pattern is used by a stepper or scanner) formed on quartz is reduced and transferred to a wafer by using a reduction projection exposure apparatus. A high-precision original design pattern is drawn by an electronic beam drawing apparatus and a so-called electronic beam lithography technology is used.
One method in electronic beam lithography is a VSB method in which variable shaped beams are used. In this method, a graphic pattern is drawn on a specimen placed on a movable stage by using, for example, electron beams that have been formed in an arbitrary shape by being passed through an opening with a first forming aperture and an opening with a second forming aperture.
In this VSB method, a pattern is formed by linking beams formed so as to have various sizes and shapes. To draw a high-precision pattern on a specimen, it is necessary to adjust an offset, which is the amount of difference in beam size, to an optimum value and set the value in the drawing apparatus.
In a conventional method of obtaining an optimum offset amount, a line pattern formed by linking divided rectangular beams with the same width is first drawn by changing the number of divisions and the offset amount to form an evaluation pattern. Next, for each offset amount, the degree (inclination) of a change in the line width of the evaluation pattern is obtained with respect to the number of divisions. Then, the offset amount at which the inclination becomes zero is obtained from each offset amount and the inclination of the change in the line width of the evaluation pattern, and this offset amount has been calculated as an optimum offset amount at which the line width does not change depending on the number of divisions.
However, the line width change amount and the number of divisions have a non-linear relationship, so there was a difference between the offset amount that was obtained by the method described above and the actual optimum offset amount. Therefore, it has been not possible to set an optimum offset amount in a drawing apparatus and it has been difficult to improve drawing precision.
Furthermore, in the conventional method described above, the evaluation pattern needs to be drawn by changing the number of divisions and the offset amount, so obtaining the optimum offset amount has taken time.